Experience has shown that in close to one hundred percent of the instances where data is available, the initial failure in satellites or similar vehicles has been in the power cells. This is so for a number of reasons.
First, the power cells, or alkaline storage cells, are the one "system" in a space vehicle which really has no back-up replacement in the event of a failure. Also, obviously, a power source cannot be engineered around as is possible with many other components of a satellite or space vehicle.
More important, however, is that the conditions in space under which a power cell must function are especially rigorous. In a typical orbit, the space craft is shielded from the sun for part of the time and must rely on its batteries for power. When the craft is in direct sunlight, it is powered by solar cells which also recharge the batteries. The time intervals involved will depend on the orbit, the speed of the craft and other factors. Further, the demands on the power cell will also vary with the equipment on board, whether the power drain is steady or sporadic, etc.
The predominant cause of power cell failure is the separators within the cells. The separators break down both chemically and mechanically. As this occurs, the degree of polarization in the cell is gradually increased which results in loss of all potential. Eventually, the cells cannot meet power requirements and the space craft loses its usefulness and operation objectives.
There are three factors which electrically affect the separators in power cells. These factors, which operate more-or-less synergistically, are the extent of the charge, the rate of charge or discharge, and the number of life cycles, i.e. charge/discharge cycles. For example, it is not uncommon in space to charge power cells to 120-130 percent of their normal capacity. It is also not uncommon to rapidly take a huge amount of power out of a power cell and then not use it for from 24 to 48 hours. Frequently, power cells in space are on a continuous slow or trickle charge which is preferable to rapid charging just as gradual discharge is preferable to a large rapid outflow of power.
The most damaging of these factors affecting separators in power cells is the number of life cycles, particularly where the discharge and/or the recharge are rapid as opposed to gradual. Other factors notwithstanding, the number of life cycles determines the life of a battery. It will be readily appreciated that the life cycle capacity of a battery utilized in space vehicles can have an economic impact in the millions of dollars.
The remaining factor which materially affects the useful life of a power cell in space is the rapid, acute temperature changes as the vehicle passes in and out of direct sunlight. While it is commonplace to provide heating means on power cells in space craft to somewhat negate cold temperatures, little can be done about the heat other than insulation of various forms. Heat, therefore, is a damaging factor to be considered in the development of new battery materials. In accordance with this invention, there is provided a separator material for power cells which is clearly superior to presently used materials in terms of its resistance to the above-identified degrading factors.